The depressive symptoms of Seasonal Affective Disorder (S.A.D.; Rosenthal et al., 1984) which reliably recur during the relatively "dark" months of the year in the Temperate Zone, appear to be treatable by exposure to bright artificial indoor light. Light Therapy offers promise as an alternative to standard medication strategies; among its advantages are rapid action within a few days, and minimal side-effects. Previous studies (in Alaska, England, New York, Oregon, Switzerland, and Washington, D.C.), have demonstrated alleviation of atypical-vegetative symptoms such as fatigue, hypersomnia, and hyperphagia, as well as typical symptoms such as depressed mood, anxiety, and work disturbance. This research, howerver, has been limited to small subject samples relative to those required in standard antidepressant medication trials, and the data show considerable variation in treatment efficacy, as well as discrepancies in the pattern of results under various lighting regimens (intensity, duration, and time-of-day of treatment). Our preliminary study of 28 S.A.D. patients, performed in November-March 1985-86, indicated a graded continuum of efficacy across treatments that included morning + evening, morning-alone, and evening-alone bright-light exposures; morning light showed an apparent strong advantage. We now propose a three-year outpatient investigation of Light Therapy in a larger group (N greater than 80 for completers), with adequate controls for established unambiguously the specific antidepressant action of light. Wake-up time, scheduling compliance, and alert state will be structured through use of a microcomputer task package positioned at the Light apparatus. The patients will be tested within a conservative two-group crossover design that includes: baseline assessment; morning light; evening light; a treatment control under dim room illumination (computer tasks alone); and withdrawal. Independent blind raters will perform clinical evaluations. The study aims to estimate reliably the proportion of S.A.D. patients who show clinically significant improvement under Light Therapy, and the magnitude of the effect relative to a set of credible comparison "placebo" conditions.